In order to prevent generation of an interference fringe on a photoreceptor for laser printers, it is a practice recently followed to roughen the surface of a conductive substrate of the photoreceptor by various means. In using a substrate with a roughened surface, however, a charge generating layer formed thereon tends to suffer from coating defects, such as cissing and spitting, and charges tend to be locally injected from the substrate into the charge generating layer to cause black dots or white areas.
It is commonly known that these problems can be solved by providing a subbing layer between a conductive substrate and a charge generating layer.
Known materials for forming the subbing layer include thermoplastic resins, such as polyvinyl acetate, polyvinyl alcohol, polyvinyl butyral, polyvinyl methyl ether, polyamides, thermoplastic polyesters, phenoxy resins, casein, gelatin, and nitrocellulose; and thermosetting resins, such as polyimide, polyethyleneimine, epoxy resins, melamine resins, phenolic resins, and polyurethane resins.
However, formation of a subbing layer comprising these known resins often causes a reduction in sensitivity, deterioration of electrical-characteristics on repeated use, such as an increase in residual potential, and deterioration in image quality, such as image defects.
To overcome these problems arising from formation of a subbing layer, it has been proposed to use an organometallic compound as a main component of a subbing layer thereby inhibiting development of coating defects or image defects without inducing a reduction in sensitivity and an increase in residual potential, as disclosed in JP-A-61-94057 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
On the other hand, various proposals have been made on an organic electrophotographic photoreceptor comprising an organic photoconductive substance and a binder resin and having a laminate structure composed of a charge generating layer and a charge transporting layer, each of which performs the respective function. Known materials for the charge generating layer include polycyclic quinone pigments, perylene pigments, indigo pigments, bisimidazole pigments, quinacridone pigments, phthalocyanine pigments, monoazo pigments, biszao pigments, trisazo pigments, and other polyazo pigments. Known materials for the charge transporting layer include benzidine compounds, amino compounds, hydrazone compounds, pyrazoline compounds, oxazole compounds, oxadiazole compounds, stilbene compounds, and carbazole compounds. Studies have also been given to combinations of these charge generating materials and charge transporting materials. With respect to combinations using phthalocyanine pigments, for example, refer to JP-A-57-54942, JP-A-60-59355, JP-A-61-203461, JP-A-62-47054, and JP-A-62-67094.
Under these circumstances, when the conventional organic charge generating materials are used in electrophotographic photoreceptors for copying machines, laser printers, LED printers, etc., the photoreceptors still have problems waiting for solution, such as poor chargeability and poor stability on repeated use.
In general, an electrophotographic photoreceptor should satisfy the following conditions for obtaining high sensitivity and stable electrophotographic characteristics on repeated use. (1) The charge generating material should generate charges efficiently in response to absorbed light. (2) The thus generated positive holes should be smoothly injected from the charge generating layer into the charge transporting layer. (3) The generated electrons should efficiently be injected from the charge generating layer to the subbing layer and exchange charges with the conductive substrate via the subbing layer thereby to inhibit accumulation of charges in the charge generating layer while preventing injection of positive holes from the conductive substrate.
That is, even if a charge generating material capable of efficiently generating charges and a charge transporting material capable of transporting the charges at a high speed are combined to satisfy the conditions (1) and (2), high sensitivity and stable electrophotographic characteristics on repeated use can never be assured unless a combination of a charge generating material and a subbing layer fulfills the condition (3).
An ideal electrophotographic photoreceptor would satisfy not only the above-mentioned conditions but all the other requirements, such as electrophotographic characteristics, e.g., sensitivity, accepted potential, potential retention, potential stability, residual potential, and spectral characteristics; mechanical durability, e.g., abrasion resistance; and chemical stability against heat, light, discharging products, and the like. However, it is very difficult to choose a combination of materials which satisfy all these requirements. A combination of a charge generating material and a subbing layer which satisfies the above conditions sufficiently has not yet been obtained.
Turning attention to production of an electrophotographic photoreceptor, solvents to be used in successive formation of a subbing layer, a charge generating layer, and a charge transporting layer on a substrate should be selected carefully. That is, a solvent to be used for formation of a layer should be capable of dispersing or dissolving all the materials for the layer to provide a composition suitable to be applied and, at the same time, must not disturb the structure of a lower layer.